There is known a stirling engine provided with a piston for gas lubrication between the piston and a cylinder (for example, see Patent Document 1). Further, for example, Patent Document 2 discloses a technique which may be relevant to the present invention, as there is provided with a moisture absorption device. Furthermore, for example, Patent Document 2 discloses technique which may be relevant to the present invention, as there is provided with a hygrometer.